To attempt the isolation of new type C viruses, especially from human tissues, using the same techniques that have led us to the isolation of other type C viruses from various non-human primates, such as baboons and gibbon apes. Various biochemical and immunological assays will be used to characterize and identify any new isolates. The apparent generality of trans-species infections of type C viruses raises the possibility that these viruses may cause neoplasia when they are in the process of being transferred from one species to another. Since there are many closely related animals available that either possess or lack virogene sequences, we intend to test this possibility by inoculating the appropriate animals with type C viruses. Since type C viruses are present in the DNA of all higher primates, we intend to use the baboon type C virus as a tool to study the evolutionary relationships among the various primate species. Since the rate of evolution of virogene sequences in the primates is approximately three times faster than that of the overall cellular DNA, closely related species can more easily be classifed phylogenetically using H3- DNA probes prepared from the baboon type C virus.